In image forming apparatuses such as copying machines and printers, the processing speed constitutes an important factor that determines the performance of the apparatus. In this regard, developments have been made to meet the demand for faster processing speed. For example, the print processing capability of “high-speed machines” has rapidly increased to 100 to 120 sheets/minute (A4-size paper, lateral feed) as compared with 50 to 70 sheets/minute (A4-size paper, lateral feed) only a few years ago. Under these circumstances, the applicable areas of such high-speed machines, from the standpoint of processing speed, now include near-printing, outside the domain of the image forming apparatus. It is therefore necessary in such high-speed image forming apparatuses to stably transport printing paper at high speed while ensuring good print quality.
In conventional image forming apparatuses, the transfer belt system has been pervasive as the system suited for high-speed processing. In image forming apparatuses adapting the transfer belt system, a printing paper electrostatically attracted to a surface of a transfer belt is transported so that a toner image formed on a surface of a photoreceptor is transferred onto the printing paper, as described in Patent Publication 1. The printing paper on the transfer belt is then sent to a fixing device, where the toner image on the printing paper is fixed thereon.
Patent Publication 1: Japanese Laid-Open Patent Publication No. 309479/1995 (Tokukaihei 7-309479, published on Nov. 28, 1995)
In a transfer step in which the toner image on the surface of the photoreceptor is transferred to the printing paper, the printing paper easily sticks to the surface of the photoreceptor and winds around the photoreceptor. This is caused by the charge on the printing paper, which is generated by the friction with various rollers transporting the printing paper, or by the transfer electric field which accumulates on the printing paper in the transfer step, among other things.
As a countermeasure, the image forming apparatus is provided with a stripping claw that forcibly strips the printing paper from the surface of the photoreceptor. In this way, a paper jam is prevented that occurs around the photoreceptor when the printing paper does not naturally strips from the surface of the photoreceptor.
Meanwhile, the image forming apparatus is designed so that the printing paper naturally strips from the surface of the photoreceptor, not forcibly with the stripping claw, taking into account such factors as the tendency of the charged printing paper to wind around the surface of the photoreceptor, the stiffness of the printing paper, the curvature of the photoreceptor, and the transport speed of the printing paper, for example. This is to prevent the stripping claw from contaminating a leading edge of the printing paper.
Specifically, in order to strip the printing paper from the surface of the photoreceptor, the stripping claw is disposed with its front end in contact with the surface of the photoreceptor. Owning to this configuration, the toner remaining on the surface of the photoreceptor easily adheres to the front end of the stripping claw. When the leading edge of printing paper is brought into contact with the front end of the stripping claw, the toner adhering to the stripping claw adheres to the leading edge of printing paper and contaminates the printing paper. Such contamination at the leading edge of the printing paper caused by the contact with the stripping claw can be avoided by naturally stripping the printing paper from the surface of the photoreceptor, before the printing paper reaches the stripping claw. The stripping claw is therefore provided as assisting means for forcibly stripping the printing paper from the surface of the photoreceptor, when the printing paper does not naturally strips.
However, despite the design that allows the printing paper to naturally strips from the surface of the photoreceptor, toner contamination occurs frequently by the contact between the leading edge of the printing paper and the stripping claw. This leads to deterioration of printed image quality as seen in conventional image forming apparatuses.